Fluid catalytic cracking (FCC) is an important process for the secondary processing of the crude oil, and plays a very important role in petroleum refineries. In the catalytic cracking process, heavy fractions such as vacuum distillate or heavier components such as residual oil are subjected to a reaction in the presence of catalyst to convert into high additional value products such as LPG, gasoline, and diesel oil. In this process, it is usually required to use a catalytic material with high cracking activity. Micro-porous zeolite catalytic material, due to the good shape-selective catalytic performance and the high cracking reaction activity, is widely used in the petroleum refining and processing industry. Since the Y zeolite was firstly used in 1960's, it has been the major active component in the catalytic cracking catalyst. Among others, the rare earth modified Y zeolite has a remarkable effect on improving the acidity and the structure stability of the zeolite.
In order to effectively utilize the rare earth resource and reduce the production cost for zeolites, especially for those having high rare earth content, it is requisite to increase the rare earth utilization in the modification and reduce the loss of rare earth, while the cracking performance of the zeolite should be maintained and the activity-stability of the zeolite should be increased as much as possible.
The currently used active component for reducing the olefin content in gasoline is mostly the rare earth-containing Y zeolite. References can be made to CN1317547A, CN1506161A, CN101537366A, CN1436727A, CN1382631A, CN101823726A, CN100344374C, CN1053808A, CN1069553C, CN1026225C, and CN101147875A.
For example, CN101147875A discloses a catalytic cracking catalyst, which contains a high rare earth ultra stable Y zeolite. Said high rare earth ultra stable Y zeolite is prepared as follows. An ultra stable Y zeolite used as starting material is mixed with an acid. The mixture is stirred, washed and filtered. To the mixture is added a solution of rare earth salt to conduct an exchange. Then the mixture is washed, filtered and dried. The product of the ratio of the strength I1 of the peak at 2θ=11.8±0.1° to the strength I2 of the peak at 2θ=12.3±0.1° (I1/I2) in the X-ray diffraction spectrogram of said high rare earth ultra stable Y zeolite and the rare earth content in the zeolite (RE2O3%) is not higher than 40.
The rare earth-containing Y zeolite can be prepared through a one-exchange-and-one-calcination procedure (the ion exchange is conducted once and the high temperature calcination is conducted once, references can be made to, for example, CN1436727A, CN101823726A and CN100344374C), or a two-exchange-and-two-calcination procedure (the liquid-phase rare earth ion exchanges are conducted twice and the high temperature calcinations are conducted twice, references can be made to, for example, CN1506161A and CN101537366A).
For either the prior two-exchange-and-two-calcination procedure or the prior one-exchange-and-one-calcination procedure, in the preparation of the rare earth-containing Y zeolite, the amount of rare earth in the product is usually lower than the total fed amount of rare earth. Even if many rare earth ions are located in sodalite cages, it is inevitable that some rare earth ions are still present in super cages. The rare earth ions in super cages can be backwashed off in the subsequent washing procedures, resulting in the loss of rare earth and the decrease of the rare earth utilization.
In the current industry, for saving the production cost and increasing the product efficiencies, upon the preparation of the Y zeolite having high rare earth content, a two-exchange-and-one-calcination procedure is mostly used. That is to say, after the first calcination, the rare earth exchange is conducted once more, but the second calcination is not done. Due to the lack of the second calcination, the effective migration of rare earth ions cannot be achieved, and most of rare earth ions are still present in super cages. The rare earth ions in super cages can be backwashed off in the subsequent washing procedures, also resulting in the decrease of the rare earth utilization.